This invention relates to a method of correcting the image output of a contact type image sensor used for the image input section of a facsimile, image scanner or the like, and more particularly to a crosstalk cancelling method of cancelling or eliminating an effect of crosstalk attributing to matrix wiring in an image sensor in which the photo sensing element forming the image sensor is divided into a plurality of blocks, and a switching element is utilized to perform a matrix drive for each block.
A contact type image sensor is made up of a photo sensing array having a plurality of photo sensing elements arranged in a line, and a drive circuit for driving the photo sensing arrays. Electric charges induced in the photo sensing elements are applied to one output line in a time-series mode by means of a switch which selects the photo sensing elements one after another. In order to drive the plurality of photo sensing elements forming the photo sensing array, it is necessary to provide a number of driving IC chips. In order to eliminate this difficulty, a TFT driven image sensor has been proposed in the art in which processing is carried out in a parallel mode with a-Si thin film transistor (hereinafter referred to as "TFT", when applicable) used for switching several picture elements simultaneously, whereby the number of driving IC chips is reduced, and the manufacturing cost is reduced as much.
In the TFT driven image sensor, as shown in FIG. 5, a photo sensing element array 50 in the form of a line equal in length to the width of an original is divided into K blocks of photo sensing elements. Each block includes N photo sensing elements 51. The photo sensing elements 51 are connected to the drain electrodes of TFTs 52, respectively. The source electrodes of the TFTs 52 are connected through a matrix circuit 53 to N common signal lines 54 in such a manner that the source electrodes of the first TFTs 52 in the K blocks are connected to the first common signal lines, the source electrodes of the second TFTs 52 in the K blocks are connected to the second common signal lines, and so forth. The gate electrodes of the TFTs 52 are connected to a gate pulse generating circuit 55 so that; more specifically, in each block, the gate electrodes of the TFTs are connected to a connecting point which is connected to the gate pulse generating circuit, so that in each block, the TFTs are rendered conductive simultaneously. The charges produced by the photo sensing elements 51 are accumulated for a predetermined period of time, and then applied to the common signal lines 54 successively according to the blocks with the TFTs 52 used as charge transferring switches so as to be led to an input terminal COM of an analog multiplexer.
The operation will be described with reference to FIG. 6 which is an equivalent circuit for one picture element including one photo sensing element 51. Initially, reset switches RS1 and RS2 are closed. As a result, a reverse bias voltage (VB) is applied to a photo-diode PD forming the photo sensing element 51, and the potential (VL) of the common signal line 54 is reset to 0 V. When an original (not shown) placed on the photo sensing element array is irradiated by a light source (not shown), the light reflected therefrom is applied to the photo-diode PD, the capacitors (CD and Cgd) on the photo-diode side are charged by the photocurrent Ip produced according to luminance signals corresponding to the densities (light and dark) of the original. After the capacitors are charged for a certain period of time, the TFT 52 is rendered conductive (turned on) by a signal VG outputted by the gate pulse generating circuit to electrically connect the photo-diode PD to the common signal line 54, so that the capacitors are discharged; that is, the charge is transferred. Since the signal input to the multiplexer is of high impedance in the potential detection method, all the charge is stored in the circuit capacitors. Accordingly, the transferring of charge means the redistribution of charge between the capacitors (CD and Cgd) on the photo-diode side and the capacitors (CL and Cgs) on the common signal line side. Thereafter, the potential VL of the common signal line 54 provided after the transferring of charge is detected, and the potential of the common signal line 54 is reset by the reset switch RS2 to transfer the picture elements signal of the next block.
In the above-described TFT driven image sensor, the matrix circuit 53 is formed by a thin film forming method as shown in FIG. 7; that is, a plurality of conductors 71 are arranged using an insulating film 70 by multi-layer wiring. This structure of the matrix circuit involves the following difficulties: Since the conductors are arranged in matrix form in the multi-layer wiring, they are extended across one another, so that coupling capacitors Cc are formed at the intersections of the conductors 71. The number of intersections is (N-1) in each of the common signal lines (N is the number of common signal lines). Therefore, when the charges are transferred from the photo-diodes PD through the TFTs 52 to the inherent capacitors (wiring capacitance) CL of the common signal lines 54, the coupling capacitors Cc are also charged due to the potential differences between the common signal lines. As a result, the charges accumulated in the fixed capacitors (wiring capacitance) CL cause variable crosstalk phenomena, so that the charges provided by the photo-diodes cannot be accurately applied to the input terminal COM of the analog multiplexer.